1. Field of the Invention
The present invention relates to a retrofocus imaging lens suitable for, for example, a 35 mm camera, a medium-format camera, a video camera, and the like. More particularly, the invention relates to a retrofocus imaging lens of a rear focus system for achieving focus by moving a part, particularly, a rear-side lens group of a lens system.
2. Description of the Related Art
Conventionally, as an imaging lens of a wide angle of view having backfocus longer than a focal length, a retrofocus imaging lens is known. The retrofocus imaging lens is generally constructed by disposing, in order from an object side, a negative lens group and a positive lens group and the principal point is behind the whole lens system.
In a lens of this kind, by disposing a lens having a strong negative refractive power on the front side, an image can be captured to a wide angle of view, and a peripheral light amount can be increased. However, on the other hand, the lens has a drawback such that a large negative distortion is caused.
A retrofocus imaging lens has, generally, an asymmetric lens configuration as a whole in which a lens group having a negative refractive power is disposed in the front part and a lens group having a positive refractive power is disposed in the rear part, so that an amount of occurrence of various aberrations such as spherical aberration, comatic aberration, distortion, and astigmatism is large. There is a problem such that it is difficult to satisfactorily correct the various aberrations in total. Further, in the retrofocus imaging lens, in order to widen the angle of view of imaging while maintaining long backfocus, the negative refractive power on the front side has to be enhanced. In this case, to correct various aberrations which occur accordingly, the number of lenses increases. As a result, problems such as increase in the size of lenses, increase in cost, and occurrence of ghost which is hazardous to imaging occur.
In order to solve occurrence of the negative distortion, it is sufficient to use a negative lens having a high refractive index as the negative lens in the front-side negative lens group. However, a glass material with a high refractive index causes large dispersion, so that a chromatic aberration of negative magnification easily occurs. In the retrofocus imaging lens, by assuring a sufficient airspace between the negative front group and the positive rear group, a clear difference can be created between an incident height of an axial ray and that of an off-axis ray in each lens surface. The retrofocus imaging lens has therefore an element that the flexibility in aberration correction is increased. However, clear separation between the front and rear groups causes increase in the size of the whole system and increase in the diameter of the front lens.
Examples of conventional retrofocus wide-angle lenses are disclosed in Japanese Unexamined Patent Application Publication Nos. Sho 62-35089 and Hei 9-113800. Each of the lenses disclosed in the publications has front and rear lens groups sandwiching a stop. The converging action of the rear-side lens group is stronger than that of the front-side lens group, and a negative distortion is not fully corrected.
As a focusing method of a retrofocus imaging lens, a rear focus system of moving a rear group in a lens system can be considered. Generally, an amount of extending a focus lens group (hereinbelow, simply called a xe2x80x9cfocus groupxe2x80x9d) in the lens of the rear focus system is smaller as compared with an overall focus system in which the whole lens is extended. The focus group is relatively small and light and focus can be achieved with a small driving force, so that the rear focus type lens is suitable for a camera having an automatic focus detecting device, or the like. Although there are such advantages, the rear focus type lens has a problem such that aberrations fluctuate when the focus group is moved more as compared with the overall focus system and it is therefore difficult to satisfactorily correct the aberrations throughout an object distance.
Conventional rear focus type lenses are disclosed not only in Japanese Unexamined Patent Application Publication No. Hei 9-11380 but also Japanese Unexamined Patent Application Publication Nos. Sho 59-216114 and Sho 61-140910. Each of the lenses disclosed in the publications has a configuration that relative importance is placed on the focus group in comparison with the lens overall length and the total number of lenses, which is not preferable from the viewpoint of driving at the time of focusing. Particularly, in the case of using the lens in a camera having a large image size such as a medium-format camera, the size and weight are increased and, in addition, the load on the driving system becomes heavy, so that it is not preferable. Japanese Unexamined Patent Application Publication No. Hei 5-119254 discloses a technique of achieving focus only by the rear side of a stop. According to the technique, however, the focusing mechanism is complicated and the number of lenses is large. Techniques enabling focus to be achieved only by moving a part of lenses for the purpose of improving drivability are also disclosed in Japanese Unexamined Patent Application Publication Nos. Sho 55-147607 and Sho 58-202414 and the like. These techniques disclosed in the publications have a problem such that correction of aberrations only by a focus group is not sufficient, and the aberrations fluctuate largely due to movement of the focus group.
In the case of using a lens shutter in any of the above-described lenses in the publications, there is no space to dispose a mechanism for the lens shutter around the position of the stop. There is a problem such that when the space around the stop is sufficiently assured, the various aberrations cannot be satisfactorily corrected.
The present invention has been achieved in view of the above problems. Its first object is to provide a retrofocus imaging lens in which various aberrations such as a negative distortion are satisfactorily corrected. A second object of the invention is to provide a retrofocus imaging lens having a space sufficient for using a lens shutter and, particularly, having a sufficient space from a mechanism viewpoint so that a focus group does not interfere with a lens shutter mechanism at the time of achieving focus in accordance with a rear focus system.
A retrofocus imaging lens according to a first aspect of the invention includes in order from an object side: a first lens group having a positive refractive power as a whole constructed by a front-side first lens group having a negative refractive power as a whole and a rear-side first lens group having a positive refractive power as a whole; and a second lens group having a positive refractive power as a whole. Each of the front-side first lens group and the rear-side first lens group includes one or more positive lens and two negative lenses. The second lens group includes, basically, one or more positive lens and one or more negative lens. Further, a conditional expression (1) is satisfied.
xe2x88x921.25 less than f1a/f1b less than xe2x88x920.75xe2x80x83xe2x80x83(1)
where f1a denotes a focal length of the front-side first lens group, and f1b represents a focal length of the rear-side first lens group.
In the retrofocus imaging lens according to the first aspect of the invention with the basic configuration, desirably, the front-side first lens group is constructed of four lenses which are, in order from the object side, for example, a positive lens, two negative meniscus lenses each having a convex surface directed to the object side, and a positive lens, or constructed of three lenses which are, in order from the object side, two negative meniscus lenses each having a convex surface directed to the object side and a positive lens and, further, a conditional expression (3) is satisfied.
0.35 less than fn/f1a less than 0.65xe2x80x83xe2x80x83(3)
where f1a denotes a focal length of the front-side first lens group and fn denotes a synthetic focal length of the two negative meniscus lenses in the front-side first lens group.
In the retrofocus imaging lens according to the first aspect of the invention with the basic configuration, preferably, the rear-side first lens group is constructed of, in order from the object side, for example, a biconvex positive lens, a negative meniscus lens having a convex surface directed to the image side, and a negative lens having a concave surface directed to the image side, or constructed of, in order from the object side, a negative meniscus lens having a convex surface directed to the object side, a biconvex positive lens, and a negative lens having a concave surface directed to the image side. The two lens components from the object side may be separated or take the form of a cemented lens.
A retrofocus imaging lens according to a second aspect of the invention has, basically, in order from an object side: a first lens group having a positive refractive power as a whole constructed by a front-side first lens group having a negative refractive power as a whole and a rear-side first lens group having a positive refractive power as a whole; and a second lens group having a positive refractive power as a whole. The front-side first lens group includes two or more negative lenses, the rear-side first lens group includes one negative lens and two or more positive lenses, and the second lens group includes one or more positive lens and one or more negative lens. Further, the above-described conditional expression (1) is satisfied.
In the retrofocus imaging lens according to the second aspect of the invention with the above basic configuration, preferably, the front-side first lens group is constructed of, for example, two negative meniscus lenses each having a convex surface directed to the object side, and the rear-side first lens group is constructed of, in order from the object side, a biconvex positive lens, a negative meniscus lens having a convex surface directed to the image side, and a positive meniscus lens having a convex surface directed to the image side. Further, it is desirable that a conditional expression (6) is satisfied.
xe2x88x925.5 less than f5/f1a less than xe2x88x924.0xe2x80x83xe2x80x83(6)
where f5 denotes a focal length of the fifth lens from the object side and f1a denotes a focal length of the front-side first lens group.
The retrofocus imaging lens according to the first and second aspects of the invention preferably further includes a rear focus mechanism for allowing the second lens group which functions as a focusing lens to travel toward the object along an optical axis, thereby allowing an image focal point position to remain unchanged even though a distance from the object changes from infinite to closest range, wherein a conditional expression (2) is satisfied.
2.0 less than f1/f less than 6.0xe2x80x83xe2x80x83(2)
where f1 denotes a focal length of the first lens group and f represents an overall focal length of the retrofocus imaging lens.
The retrofocus imaging lens according to the first and second aspects of the invention with the basic configuration, preferably, further includes: an aperture stop disposed between the first and second lens groups; and a flare-cut aperture disposed between the aperture stop and the second lens group, the flare-cut aperture traveling together with the second lens group when focusing, thereby suppressing a comatic flare caused by a part of oblique incident light, the part being far from the optical axis.
In the retrofocus imaging lens according to the first and second aspects of the invention, preferably, the second lens group is constructed of, in order from the object side, for example, a positive lens having a convex surface directed to the image side, a negative lens having a concave surface directed to the object side, a positive meniscus lens having a convex surface directed to the image side, and a positive lens having a convex surface directed to the image side. Desirably, a conditional expression (4) is satisfied, and at least one of dispersions xcexd2xe2x88x92i (i=3 or 4) satisfies a conditional expression (5)
1.0 less than f1/f2 less than 4.0xe2x80x83xe2x80x83(4)
60 less than xcexd2xe2x88x92ixe2x80x83xe2x80x83(5)
where xcexd2xe2x88x92i denotes a dispersion for a d-line of light of the i-th lens from the object side in the second lens group.
In the retrofocus imaging lens according to the first and second aspects of the invention, with the above-described basic configuration, various aberrations such as negative distortion are satisfactorily corrected. Particularly, like the retrofocus imaging lens according to the first aspect, one or more positive lens is added to the negative front-side first lens group and the negative lens is added to the positive second lens group which is positioned on the rear side of the first lens group, thereby enabling negative distortion to be satisfactorily corrected.
In the basic configuration of the retrofocus imaging lens according to each of the aspects, by properly employing the desired configuration, more satisfactory aberration correction and the performance suitable for the rear focus system are obtained. Particularly, by satisfying the conditional expression (2) in the configuration of the rear focus system using the second lens group as a focus group, a relatively long focal length of the first lens group on the front side of the focus group can be obtained, and an outgoing light flux can be properly set close to an afocal state. Thus, fluctuations in aberrations when the focus group is moved are suppressed. At this time, the image formation magnification in the focus group increases, and the movement amount in focusing may be relatively small. In the case of using a lens shutter near the position of the stop since the movement amount at the time of focusing is small, a sufficient space can be assured from a mechanism viewpoint so that the focus group does not interfere with the lens shutter mechanism at the time of focusing.
Other and further objects, features and advantages of the invention will appear more fully from the following description.